Proliferative kidney disease (PKD) is one of the most economically important diseases among commercially reared rainbow trout (Oncorhynchus mykiss) in Europe and causes significant losses among Pacific salmon and rainbow trout populations in western North America. The parasite that causes PKD is a poorly understood myxosporean (PKX), presumed but not yet proven to be a member of the family Sphaerosporidae, genus Sphaerospora. The disease occurs in both cultured and feral populations of salmonids that come into contact with a 20-25 µm waterborne infective stage. The disease is often seasonally dependent occurring at water temperatures above 15 °C in the summer and autumn months of the year. Mortality in uncomplicated cases of PKD is generally 20% or less but often secondary pathogens or unfavourable environmental conditions coincide with peak periods of PKD and mortalities can reach 95-100%. In any surviving fish that have experienced a full clinical episode of the disease, a strong acquired immunity develops.